Changing electrical power system needs in the last decade have placed a more stringent demand for the rapid transfer of power feeding a large motor bus from a main source of power, on which a fault has occurred, to an auxiliary source which will not power the motor or motor bus. Various methods are currently used for providing such transfer of power from the main source to the auxiliary source. One method has been to utilize two breakers, one coupling from the motor bus to the main source of power and the second breaker coupling from the motor bus to the auxiliary source of power. Closing of the second breaker is initiated, and the tripping or opening of the first breaker is accomplished by the same electrical contact. This requires the closing time of the second breaker to always be slower than the tripping time of the first breaker. If for some reason the foregoing does not occur in the exact time sequences stated, fault current can flow momentarily through the station service transformers resulting in the dramatic failure of the transformers.
A second method has been to initiate closing of a second breaker with a contact on the first breaker. However, in this method the transfer time may often be so long as to produce a worst case phase angle between the induced motor voltage and the new source. This can produce damaging stress on the motor shaft.
A third method lets the motor voltage decay to 0.25 of the rated unit voltage. At this value, the worst case current inrush is within the motor design limit. The time required for this decay to take place, however, may be so long that an undesired or damaging plant shutdown may occur.
Note as further prior art, U.S. Pat. No. 3,491,248, entitled Power Transmission Line Switch Control System, issued to Robert W. Beckwith on Jan. 20, 1970. U.S. Pat. No. 3,491,248 discloses a system for controlling the automatic closure of the contacts of a selected circuit breaker connected between a bus and a power line. The voltages on the bus and line sides of the selected breaker are connected to a controller unit and the controller effects automatic closure of the selected breaker if the proper voltage, frequency and phase matching conditions exist between the bus and power line. While the device of U.S. Pat. No. 3,491,248 provides satisfactory service for a number of applications, present-day requirements dictate a need for much faster operating transfer relay systems.
Surveys of the market place indicate that a fast transfer relay circuit is needed in a number of applications to quickly establish the closing of the breaker to an alternate source of station power when fault causes tripping of the main circuit breakers out of a generator plant. One specific application is, for instance, in a nuclear plant wherein the alternate source must be connected to provide power to the system within seconds otherwise the plant will automatically shutdown. In a coal fired plant, the critical time may be even shorter since slowing down of the induced draft fan can cause a boiler explosion due to the inertia of air coming into the boiler.
Accordingly, prior art methods and devices do not meet certain present day demands. More specifically, the power transfer relay circuitry of the invention meets the requirement for a system providing safe, high-speed synchronous transfer of power to a large rotating electrical machine from one source of power to another source.
Refer now to FIG. 1 of the drawings which shows a simplified, typical generating plant complex 9. Under normal conditions, the generator 21 is used to provide power through the main transformer and a first circuit breaker 11 to the motor bus 15. A fault in the system 18, such as a fault in the main transformer 16 which results in the emergency shutdown of the generator would, of course, also cause breaker 11 to open and interrupt power to the motor bus 15. As mentioned above, it is desirable to restore power to the motor bus as quickly as possible without damaging the rotating equipment. This can be accomplished by closing a second circuit breaker 12; providing a means is available to quickly check synchronism between an auxiliary power source or system 17 and the varying frequency of the induced voltage of the rotating equipment connected to the motor bus 15. The inventive system shown in dotted lines and labeled 10, provides a means of verifying the synchronization of the auxiliary source with the frequency of the motor bus, and provides a means for properly initiating the closing of the breaker 12. The inventive system includes the capability of predicting the point in time when the proper phase relation between the voltages will exist, and of initiating breaker closing so that safe and synchronous restoration of the service results. More basically, the inventive transfer relay circuitry causes contacts of breaker 12 to close when the phase difference between the motor bus frequency and the auxiliary source frequency is very nearly 0.degree., to thereby minimize transient power flow. Thus, the present invention automatically effects the closure of a circuit breaker to connect an auxiliary system or source of power, rapidly and safely, to a motor bus when a fault occurs in the main power source or system.